In addition to what NEDOC says it helps in catching them if they are the only sunfish in the pond. I catch one rarely in my main pond where there are also BG along with some GSF and hybrids. I can catch them much better in a couple of ponds where they are the dominant sunfish, although they can still be hard to catch.
My theory is three fold why they are so hard to catch. First in most ponds are just the sheer numbers and odds. The RES population might only be one RES to a hundred or maybe even a thousand BG. So by nothing more than odds it is much less likely to catch a RES. Second they are not nearly as aggressive to bite an artificial bait. So if you have a hundred hungry BG around your cast and one finicky RES, which is more likely to take the bait? Third their range of preferred food items is less than the BG. So it takes a narrower group of baits they are willing to take.
I seem to be able to catch the small and medium size RES in RES only ponds reasonably well on some days. The big ones are much harder for me to entice to bite a hook.
RES information thread and links to other threads about RES 
